1. The Field of the Invention
The present invention relates to a sloped contact opening in a polysilicon layer, and a method for etching the sloped contact opening which is selective to oxide. More particularly, the present invention is directed to a vertically oriented capacitor formed within a sloped contact opening in a layer of polysilicon with an underlying oxide etch barrier layer, and a corresponding method for forming a vertically oriented capacitor within the sloped contact opening.
2. The Relevant Technology
Integrated circuits are being designed on an increasingly smaller scale. The smaller scale is necessary to make the integrated circuits more efficient, and aids in constructing the integrated circuits at a lower cost. These are particularly desirable characteristics in certain areas such as DRAM fabrication. One difficulty, however, in fabricating integrated circuits such as DRAM memory modules is in allocating sufficient surface area for the many capacitors that are required therein.
Traditional MOS capacitors are horizontally oriented and require a high amount of surface area. To overcome this problem, the prior art has used various forms of vertically oriented capacitors, including "stacked" capacitors. Such capacitors have a cylindrical shape with an inner plate made of a conducting material such as polysilicon, the inner plate being surrounded by a dielectric such as silicon dioxide, and an adjacent outer capacitor plate also made of a conducting material. The stacked capacitor is typically formed on a silicon substrate. A region of the substrate known as the active region is located at the bottom of the contact opening within which the vertically oriented capacitor is formed. The active region connects to the inner plate of the capacitor, which connection connects the capacitor with other semiconductor devices formed on the silicon substrate. One example of a stacked capacitor is given in U.S. Pat. No. 4,951,175 to Kei Kurosawa.
A contact opening 28 for a stacked capacitor is seen in FIG. 1. An underlying substrate 10 has formed thereon an active region 12, which is typically a source or drain region of a transistor. Above active region 12 is contact opening 28 formed in a polysilicon layer 26. Two field oxide "bird's beaks" isolation regions (not shown) can be located to either side of active region 12.
FIG. 2 is a further construction of a stacked capacitor within contact opening 28 seen in FIG. 1. An inner capacitor plate 36 is typically formed within contact opening 28 and is intended to make electrical contact with active region 12. A dielectric layer 37 is deposited over inner capacitor plate 36 and a outer capacitor plate 38 is deposited over dielectric layer 37.
It is undesirable that contact opening 28 make contact one or more bird's beak isolation regions which may be located on either side of active region 12. It has proven difficult, however, to center contact opening 28 directly above active region 12, especially when active region 12 is a small area. Thus, it is undesirable that inner plate 36 formed within contact opening 28 overlap one of the bird's beak isolation regions that may be on either side of active region 12. Such an overlap may cause a leakage of the capacitor. Leakage from capacitors can cause failure in the particular circuit being formed. In the particular case where the capacitor is part of a memory cell of a DRAM memory module, a leaky capacitor will be unable to maintain charges between refresh states. This causes data corruption and thus a defect condition.
A further problem encountered with the structure of FIG. 2 is that polysilicon layer 26 is formed as the sidewall seen in FIG. 1 and 2 to be as thin as possible for greater device densities. Thus, the sidewalls of polysilicon layer 26 have a much greater dimension in the Y direction than in the X direction seen in FIGS. 1 and 2. During the rigors of fabrication, the sidewalls of polysilicon layer 26 can be lifted completely off of underlying silicon substrate 10 due their thinness. At the minimum, the sidewalls of polysilicon layer 26 can be bent or damaged. Thus, a structure with greater structural rigidity is desirable.
Accordingly, a method is needed for creating capacitors with smaller horizontal contact area to the underlying substrate. Particularly, a method is needed whereby a vertically oriented capacitor can be formed such that the capacitor can be easily located directly over an underlying active region of modest proportions without overlap into adjacent regions. Such a capacitor must also be relatively structurally rigid in order to overcome the problems discussed above.